Corrosion, erosion, fatigue, creep are examples of degradation mechanisms that limit the life time of structures and cause significant economic impact on many industries. The prediction of life time of structures is often not precise enough and inspection methods are regularly applied to structures to determine its health condition. Ultrasonics is one of the most valuable non-destructive testing methods to evaluate the condition of structures exposed to degradation mechanisms.
Structural health monitoring (SHM) of components of power plants, oil & gas and aerospace industries, such as pipes and other vehicle parts having respective contoured surfaces is desirable, but tricky. Access to the structural parts to be inspected may be difficult due, for example, to safety concerns for inspectors, or to a limited access to the structure surface due to thermal insulation, adjacent mechanisms, electrical, pneumatic or hydraulic control lines, etc. Furthermore, once access is provided, installing a sensor in a way that provides adequate contact between a piezoelectric material and the component to be tested, may be difficult. Also, the inspection may require the shutdown of a running process. Therefore permanently or semi-permanently installed transducers are often preferred, especially in transducers that permit a state of the structure to be sensed at any time (on demand) by electronic request. The time between inspections is typically chosen in order to provide accurate prediction of the remaining life time of the structures. Therefore permanently attached transducers with easy electronic interrogation of the sensor are in demand to make shorter and less costly interrogation of the condition of the structure.
Sensors for structural health monitoring are often required to operate in harsh environments as for example, at high temperatures. Therefore the permanently attached transducers may also be required to maintain its performance for long periods of time, at high temperatures. High temperatures transducer configurations have been proposed using a delay line and cooling of transducers as for example, in U.S. Pat. No. 7,185,547 to Baumoel, entitled “Extreme Temperature Clamp-on Ultrasonic Flowmeter Transducer”. The delay lines and cooling significantly limits the value of such transducers for widespread structural health monitoring. Therefore it is required to develop transducers composed of materials that can maintain a significant proportion of its essential properties at high temperatures.
Similar techniques have been used to separate ultrasonic transducers from high temperature surfaces and the separation, though perceived to be required for operation of the sensor, impairs the use of the sensor, and occupies a lot of space around the component.
Other examples of patents in this field are: U.S. Pat. No. 3,781,576, to Runde, et al., entitled High temperature Ultrasonic Transducer; U.S. Pat. No. 4,392,380, to Caines entitled High Temperature Pressure Coupled Ultrasonic Waveguide; U.S. Pat. No. 4,567,770 to Rumbold et al., entitled Ultrasonic Transducer Apparatus and Method for High Temperature Measurements; U.S. Pat. No. 4,738,737 to Runde et al., entitled Method of Using a High Temperature Ultrasonic Couplant Material; U.S. Pat. No. 4,783,997, to Lynnworth entitled Ultrasonic Transducers for High Temperature Applications; U.S. Pat. No. 5,325,012 to Sato et al., entitled Bonded Type Piezoelectric Apparatus, Method for Manufacturing the Same and Bonded Type Piezoelectric Element; U.S. Pat. No. 5,886,456 to Stubbs et al., entitled Ultrasonic Transducer and Ultrasonic Detection and High Temperature Processing Systems Incorporating Same; and U.S. Pat. No. 7,743,659 to Kearns et al., entitled Structural Health Monitoring (SHM) Transducer Assembly and System.
“Flexible Ultrasonic Transducers” by Kobayashi, M., Jen, C. K, and Levesque, D., IEEE Trans. on Ultrasonics, Ferroelectrics and Frequency Control, vol. 53, no. 8, August 2006 teaches that the flexible ultrasonic transducers (FUTs) are applicable to in-situ SHM, NDT and on-line diagnosis. To demonstrate the flexibility and operability of such FUTs when flexed, one experiment was performed on a thick pipe at room temperature and at elevated temperatures. As shown in FIG. 1, and FIG. 2, a viscous oil ultrasonic couplant was applied on the pipe at the location of testing, it was covered with a 75 micron stainless steel foil, and a PZT piezoceramic film, which was topped with a silver paste, to serve as an electrode. The pipe itself served as the ground. A mechanical holder is provided to maintain the FUT in place.
While the mechanical holder of Kobayashi et al. encircled a pipe, it was not a clamp for the FUT as the term is used in the present art, and the active area of the FUT is substantially free of pressure as there is a wide opening in the clamp surrounding the FUT, as is shown in FIGS. 1 and 2. The purpose of the clamp is just to prevent slipping of the FUT on the oiled, rounded, surface. Also in this previous art, the electrical connections are made by spring-loaded pins that are not practical for real industrial applications.
Other previous art that uses some kind of flexible transducers like U.S. Pat. No. 5,166,573 to Brown, for example, which teaches how to produce polymer-base FUTs and flexible UT arrays, but does not teach a practical assembly to attach to permanently monitor structures. Also, as the piezoelectric layers of these transducers are polymer-based, they are inherently incapable of high temperature applications.
Accordingly, there is a need for improved ultrasonic transducer assemblies to perform structural health monitoring that can be permanently installed in curved objects, can maintain performance for long periods of time, including at high temperatures and harsh environments, and can allow easy and automated electronic interrogation of the condition of the structure.